1. Field of the Invention
This invention relates to a method and system for controlling fuel vapor purging of a lean burn engine in addition to performing adaptive learning at near stoichiometric operation.
2. Background of the Invention
Engine fuel systems typically contain a canister for collecting fuel vapors produced in the fueling system. Periodically, these fuel vapors are passed through to the engine and burned during engine combustion. In this way, the generated fuel vapors are utilized by the engine and emissions are decreased.
Engines also contain adaptive learning methods to learn air flow sensor and fuel injector variations. These sensors and actuators can vary from engine to engine, and can also vary over time. These adaptive methods are used to learn this variation and take corrective action.
One approach to incorporating fuel vapor purging and adaptive learning into a lean burn engine discontinues lean operation when fuel vapor purging is requested since this allows learning of the fuel vapor concentration.
In another example, fuel vapors are periodically purged from a fuel system into an engine""s air/fuel intake. A measurement of the massive inductive fuel vapors is provided by a purge compensation signal is derived from an exhaust gas oxygen sensor output. Lean air/fuel operation is enabled when the purge compensation signal is below a predetermined value. Such a system is described in U.S. Pat. No. 5,735,255.
However, the present inventors have recognized a disadvantage with such approaches. In particular, since adaptive learning is also performed during stoichiometric operation, there is limited ability to learn both the fuel vapor concentration and the air and fuel adaptive errors. In particular, air and fuel adaptive errors, as well as changes in fuel vapor concentration, affect the engine exhaust sensors in similar ways. As such, during stoichiometric operation, it is generally possible to only perform one at a time, thereby providing only limited operating time for each. This can lead to degraded fuel vapor control, as well as degraded adaptive learning. Furthermore, fuel vapor purging performed during lean and high load operation can give degraded performance since all of the fuel vapors may not be burned during such high load lean operation.
The above disadvantages are overcome by a method for an internal combustion engine coupled to an emission control device and a sensor, the method comprises: operating the engine lean with periodic rich operating durations; during said lean operation with periodic rich operation, learning a fuel vapor amount based on the sensor, wherein fuel vapor is combusted in the engine at least during a part of the lean and rich operation; operating the engine at near stoichiometric operating conditions; and during said stoichiometric conditions, learning adaptive data.
By performing the fuel vapor purging during the lean/rich operating conditions, while performing adaptive learning during stoichiometric conditions, it is possible to provide sufficient time to achieve accurate adaptive learning and accurate fuel vapor learning. Further, an added advantage is that any errors in the fuel vapor concentration during the lean operation, thereby causing an error in the engine air-fuel ratio, provides only a minimal impact since high accuracy air-fuel ratio control is not required during lean operation (i.e., the primary emission is NOx, which is being stored in the emission control device). Still further, an added advantage is that when there is no need to perform fuel vapor purging and adaptive learning, it is possible to run the engine with a reduced number of cylinders, thus further reducing pumping losses and improving fuel economy.
In another aspect of the invention, the above disadvantages are overcome by a system. The system comprises an emission control device capable of storing NOx during at least some operating conditions; and a controller programmed to operate an engine lean at least during a light load operation; during at least a portion of said lean operation, operate the engine to induct fuel vapors; and while inducting said fuel vapors, switching the engine to stoichiometric or rich operation to react NOx stored in said emission control device.
By using an emission control device that can store NOx during certain operating conditions, one can operate lean while combusting fuel vapors without having to stop the fuel vapor purging when transitioning to stoichiometric or rich operation. Also, any changes in the fuel vapor concentration provide only a minimal impact since the engine is operating lean of stoichiometry.
In other words, the present inventors have recognized that if fuel vapors are inducted during certain lean operating conditions, emissions are less susceptible to sudden changes in the fuel vapor concentration due to fuel tank sloshing. Also, because of this reduced sensitivity, it is possible to rapidly open or close the fuel vapor purging valve, thereby allowing quicker fuel vapor purging. Further still, feedback can be used to adjust and check fuel to maintain the desired air-fuel ratio, as well as the desired engine output torque.